We report an experimental investigation of the motion bed load particles under steady and spatially uniform turbulent flow above a flat sediment grain size. Using high‐speed video imaging system, we recorded trajectories moving measured their velocity length duration flights, as well surface density particles. Our observations show that entrained exhibit intermittent composed succession periods “flight” rest. During one flight, particle may go through phases reptation, during which it moves...

An erodible bed sheared by a fluid flow, gas or liquid, is generally unstable, and forms grow. This review discusses the following issues, in light of recent literature: What are relevant dynamical mechanisms controlling emergence forms? Do they form linear instability nonlinear processes such as pattern coarsening? determines their timescales length scales, so different air water? similarities differences between aeolian subaqueous patterns? influence mode transport: load, saltation,...

Experiments are reported on the dynamics of a bed particles sheared by viscous Couette flow in an annular channel, with emphasis distributions particle velocities, durations and lengths small saltation flights, surface density moving particles. The velocity shown to decay approximately exponentially, mean value, still remains too short. Our observations are, finally, compared existing numerical experimental studies turbulent flows.

The stability of a granular bed sheared by liquid flow is investigated using an erosion-deposition model for the moving grains. Extending to arbitrary wave number previous long analysis [F. Charru and E. J. Hinch, Fluid Mech. 550, 111 (2006)], this study shows that short waves are stabilized crest-erosion mechanism absent when local particle flux assumed be in equilibrium with shear stress. cutoff length associated scales on deposition particles. This characteristic similar, although...

Experiments are reported on the formation and migration of isolated dunes in a turbulent channel flow. These have very robust crescentic shape with horns pointing downstream, similar to that barchan observed deserts at much larger scale. Their main geometrical dynamical properties studied detail, for four types grains: conditions their formation, morphology, threshold shear stress motion, velocity, erosion rate, minimum size longitudinal stripes grains hollowed by fluid streaks boundary...

A unified view is given of the instabilities that may develop in two-layer Couette flows, as a ‘phase diagram’ parameter space. This obtained from preliminary study single-fluid flow over wavy bottom, which reveals three regimes for disturbances created at each regime being characterized by typical penetration depth vorticity and an effective Reynolds number. It appears exhibits same induced perturbed interface, type instability can be associated with regime. Typical curves giving growth...

The instability of a bed particles sheared by viscous fluid is investigated theoretically. flow over the wavy first calculated, and shear stress derived. particle transport rate induced this calculated from resuspension theory Leighton & Acrivos (1986). Mass conservation then gives explicit expressions for wave velocity growth rate, which depend on four dimensionless parameters: wavenumber, thickness, length stress. mechanism given. It appears that high enough fluid-layer long-wave...

A new approach to studying ripple formation in steady flow is proposed, based on a model for the erosion and deposition of particles. Higher shear stresses at crests lead loss particles from there, which can suppress instability induced by fluid inertia. This accounts recent observations that ripples do not grow when viscosity increased, behaviour present classical an algebraic dependence particle flux bottom stress.

The motion of a spherical particle along rough bed in simple shear viscous flow is studied experimentally for wide range parameters, varying the size and density, fluid viscosity rate γ. instantaneous velocity calculated stream, transverse vertical directions, using high-speed video imaging system. It found that normalized streamwise mean U / S , where Stokes settling velocity, depends only on dimensionless μ γ/(Δ ρ g d), this relationship being independent Reynolds number Re p . This result...

The moving layer at the surface of a granular bed sheared by viscous flow has been investigated experimentally. fluid and particle velocities have measured using imaging velocimetry (PIV) tracking, respectively, with technique matched index refraction. mean velocity profiles are found to be parabolic. models Bagnold ( Phil. Trans. R. Soc. Lond . A, vol. 249, 1956, pp. 235–297) Leighton & Acrivos Chem. Engng Sci ., 41, 1986, 1377–1384) fail account for observations. A simplified model...

A linear stability analysis of a shear flow in the presence continuous but steep variation viscosity between two layers nearly uniform is presented. This instability investigated relation to known interfacial for parallel superposed fluids different viscosity. With respect this configuration, our problem depends on new parameters: interface thickness ranges.

Using a perturbation method up to the second order, equation for long waves at interface between two viscous fluids is derived plane Couette–Poiseuille flow, and moderate surface tension. The leading order Burgers equation. Higher terms take into account linear dispersive effects stabilizing effect of tension, involve more nonlinear terms. exactness coefficients this has been checked by using symmetry properties. For zero gravity, Poiseuille flow shown be stable if only velocity profile...

Abstract We report an experimental study of the turbulent flow above a barchan dune in channel, from particle image velocimetry measurements, for Reynolds numbers ranging 9000, just below threshold motion, up to 24 000, where moves. Two calculations speed-up over are compared, usual ‘same-elevation’ and more relevant ‘Lagrangian’, showing that latter is smaller by factor two. The two-layer structure disturbance – essentially inviscid outer layer inner thickness ${\delta }_{i} $ assessed. In...

Interfacial stability of two-layer Couette flow was investigated experimentally in a channel bent into an annular ring. This paper is focused on the supercritical long-wave instability which arises for broad range parameters. Above critical upper plate velocity, slowly growing long wave appears with wavelength equal to perimeter channel. Transients this were studied within theoretical frame amplitude equations obtained from interface equation. Near onset instability, unstable fundamental...

We show that in a turbulent flow transporting suspended sediment, the unsaturated sediment flux $q(x,t)$ can be described by first-order relaxation equation. From mode analysis of advection-diffusion equation for particle concentration, length and time scales dominant are shown to deposition $H U/V_{\rm fall}$ $H/V_{\rm fall}$, where $H$ is depth, $U$ mean velocity $V_{\rm settling velocity. This result expected particularly relevant case transport slowly varying flows, never far from...

The xylem vessels of trees constitute a model natural microfluidic system. In this work, we have studied the mechanism air flow in Populus xylem. vessel microstructure was characterized by optical microscopy, transmission electronic microscopy (TEM), and atomic force (AFM) at different length scales. ≈15 cm diameter ≈20μm. Flow from one to next occurs through ∼102 pits, which are grouped together ends vessels. pits contain thin, porous pit membrane with thickness 310 nm. We measured Young's...

The analysis of flow over a slowly perturbed bottom (when perturbations have typical length scale much larger than channel height) is often based on the shallow-water (or Saint-Venant) equations with addition wall-friction term which local function mean velocity. By this choice, small sinusoidal disturbances wall stress and velocity are bound to be in phase each other. In contrast, studies shorter-scale long established that lead develops between velocity, crucial destabilizing effect...

Section-averaged equations of motion, widely adopted for slowly varying flows in pipes, channels and thin films, are usually derived from the momentum integral on a heuristic basis, although this formulation is affected by known inconsistencies. We show that starting energy rather than equation makes it become consistent to first order slowness parameter, giving same results have been provided until today only much more laborious two-dimensional solution. The kinetic-energy correctly...

The formation of sand ripples in oscillating flows is thought to be due a steady streaming current which near the bottom towards crests. We present quantitative observations this mean flow over self-formed and artificial ripples, by observing displacement coloured filament after number oscillations simple situation viscous Couette flow. Confusingly, moves ‘wrong’ direction, because it follows Lagrangian calculate A complication that amplitudes experiments are not small. compare predictions...